Packagings for the packaging of liquid products such as liquid dairy products or juices are made from coated paperboard, which is termed liquid board. This is specially designed to counteract the effect of the liquid with which it is to be filled. In particular, it is the cut edges of the paperboard that have to be protected against the penetration of liquid, so-called edge penetration. For liquid dairy products, the most aggressive component of the liquid is lactic acid. The paperboard is therefore hydrophobized against the penetration of lactic acid, which is normally achieved by means of a neutral size, i.e. a size that produces optimal hydrophobicity when it is applied at pH values above 7. Typical neutral sizes comprise at least one hydrophobic, cellulose-reactive hydrophobizing agent and an anionic, cationic or amphoteric polymer, normally a cation-active starch. The, hydrophobic, cellulose-reactive hydrophobizing agent often consists of compounds of the type alkyl ketene dimers (AKD) or alkenyl succinic anhydride (ASA). These are cellulose-reactive and bind directly to the carboxyl groups of the cellulose.
For aseptic liquid board, it is also the case that the paperboard has to be sterilized/disinfected before it is filled with the liquid food. Packagings of liquid board are normally disinfected by contact with a peroxide compound such as hydrogen peroxide solution at an increased temperature, e.g. around 70° C. Here also, however, it is the case that the liquid (disinfectant solution) must not penetrate the paperboard, the most exposed part being the cut edges. Unfortunately, however, the means normally used to prevent the penetration of lactic acid, i.e. neutral size, does not protect against the penetration of peroxide compounds, such as hydrogen peroxide. The liquid board is therefore hydrophobized conventionally against the penetration of peroxide by means of a second hydrophobizing agent, normally so-called resin size. Resin size usually comprises pine resin and/or gum resin. The resins comprise resin acids and a small portion of fatty acids. Commercially available hydrophobizing agents consist of so-called fortified resins, meaning that further carboxyl groups have been introduced to increase the retention tendency. The resin size is anchored to the cellulose surface by means of alum. Cationic resin size can be used in theory without alum, but the practical effect is better with alum.
The problem with hydrophobizing paperboard with both neutral size and with resin size is that hydrophobizing takes optimal effect at different pH values for the different sizes. As stated, neutral size works best at pH values above 7, while resin size works best at pH values below 7, meaning that a compromise solution has to be applied. However, not using the optimum pH for the two sizes means that they have to be dosed in larger quantities, incurring extra costs and also problems with the leftover chemicals that circulate in the water system, giving an unstable system with foaming and flakes etc. as a result.
In U.S. Pat. No. 5,456,800, a solution is presented to the problems named, a sizing composition being used that aims to prevent the penetration of both hydrogen peroxide and lactic acid and can advantageously be applied at a neutral to alkaline pH. The sizing composition presented comprises a thermosetting resin, a cellulose-reactive hydrophobizing agent and a non-cellulose-reactive hydrophobizing agent. The resin size and wax contain resin acids and fatty acids, which, in the case of poor retention, cause foaming and act as foam stabilizers. In U.S. Pat. No. 5,456,800, problems regarding resistance to external pressure and problems regarding hydrophobizing of paperboard layers with a low density, as below, are not discussed.
In treatment to prevent liquid penetration, account must also be taken of the density of the paperboard. The liquid board usually has a middle layer (centre layer) with a lower density than the outer layers, with the aim of giving the paperboard the desired mechanical properties, such as rigidity. The middle layer functions here as a bulk-giving layer. At low density, however, the surface energy has a smaller effect, and large quantities of size have to be dosed to achieve the desired hydrophobizing effect.
In certain types of converting equipment, furthermore, the paperboard is conveyed down into a bath of hydrogen peroxide on disinfection, the paperboard being acted upon by an external pressure that corresponds to the liquid column in the bath. With external pressure, the significance of surface chemical effects is reduced, while the capillary structure acquires greater importance. It is therefore more difficult to reduce the penetration of hydrogen peroxide through the surface chemical effect when an external pressure is present.